The present invention relates to information storage disk-handling systems, and more particularly to an improved hub for use with a removable disk that allows the disk to be accurately centered with respect to a desired rotational axis. The present invention is especially well suited for use with an automatic disk-handling apparatus, one embodiment of which is disclosed in applicants' copending application Ser. No. 481,962, filed 04/04/83 (hereafter the '962 application), which '962 application is incorporated by reference herein.
In disk storage systems, information is stored on rotating disks by selectively marking or otherwise altering the characteristics of the disk surface with a pattern or sequence that relates to the information to be stored. Information is retrieved or recovered from the disk by sensing the marked pattern or sequence through appropriate means, and reconstructing the desired information from these sensed patterns or sequences. The information is stored on the disk in a data track or tracks, that may be either concentric or spiral. The amount of information that may be placed on a given disk (the "data storage capacity") is related to how closely these data tracks may be placed on the disk surface. In the case of removable disks, the minimal spacing between adjacent data tracks is, in turn, largely dependent upon the accuracy with which the rotating disk may be aligned with a desired rotational axis (the "eccentricity" of the mounted disk) as the disks are repeatedly inserted into and removed from the information system. That is, the eccentricity of the mounted disk with respect to the rotational axis determines in large part how closely the tracks may be placed together.
The above problem--of accurately centering removable disks--is especially acute with optical disk storage systems. Such systems typically use radiant energy, e.g., from a laser beam, to mark extremely small holes or areas on the disk surface on the order of a micron in diameter. In order to achieve a maximum data capacity, it is desirable that data tracks be placed as close together as possible. For example, a desired optical disk storage system may have as many as 12,500 to 15,000 data tracks per radial inch. Such a data track density requires that the disk be extremely accurately centered each time it is inserted into the disk storage system. While appropriate tracking servo systems may compensate for some of the eccentricity that may be present, see e.g., patent application Ser. No. 438,133 filed 11/01/82, assigned to the same assignee as is the present invention, it is nonetheless desirable to have a means for accurately and repeatedly centering a removable disk to within a very tight tolerance.